U.S. patent number 4,803,021 [Application Number 07/034,794] was granted by the patent office on 1989-02-07 for ultraviolet laser treating of molded surfaces.
This patent grant is currently assigned to Amoco Corporation. Invention is credited to Virupaksha K. Reddy, Dennis L. Werth.
United States Patent |
4,803,021 |
Werth , et al. |
February 7, 1989 |
**Please see images for:
( Certificate of Correction ) ** |
Ultraviolet laser treating of molded surfaces
Abstract
Ultraviolet laser treatment of molded articles effectively
removes surface coatings, such as layers of mold-release agents, in
a one-step dry process. Subsequent processing, as by bonding or
painting, provides improved and superior quality molded articles in
an economic manner.
Inventors: |
Werth; Dennis L. (Willow
Springs, IL), Reddy; Virupaksha K. (Naperville, IL) |
Assignee: |
Amoco Corporation (Chicago,
IL)
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Family
ID: |
25255573 |
Appl.
No.: |
07/034,794 |
Filed: |
April 3, 1987 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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829794 |
Feb 14, 1986 |
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Current U.S.
Class: |
264/400;
219/121.6; 219/121.85; 264/139; 219/121.68; 219/121.69; 264/80;
264/340; 264/37.1; 264/482 |
Current CPC
Class: |
B08B
7/0042 (20130101); B29C 59/16 (20130101); B29C
33/60 (20130101); B29K 2067/00 (20130101); B29C
2035/0838 (20130101); B29C 2035/0827 (20130101) |
Current International
Class: |
B29C
59/16 (20060101); B29C 59/00 (20060101); B08B
7/00 (20060101); B29C 35/08 (20060101); B29C
33/60 (20060101); B29C 33/56 (20060101); B29C
071/04 () |
Field of
Search: |
;264/22,80,37,38,39,25,340-344,334,139,317
;219/121L,121LH,121LJ,121LM |
References Cited
[Referenced By]
U.S. Patent Documents
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4220842 |
September 1980 |
Sturmer et al. |
4247496 |
January 1981 |
Kawakami et al. |
4414059 |
November 1983 |
Blum et al. |
4417948 |
November 1983 |
Mayne-Banton et al. |
4444701 |
April 1984 |
Meguiar |
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Foreign Patent Documents
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211801 |
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Jul 1984 |
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DE |
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54-117576 |
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Sep 1979 |
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JP |
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57-22208 |
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May 1982 |
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JP |
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58-199788 |
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Nov 1983 |
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JP |
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60-245643 |
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Dec 1985 |
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JP |
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60-250915 |
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Dec 1985 |
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JP |
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Primary Examiner: Thurlow; Jeffery
Attorney, Agent or Firm: Kretchmer; Richard A. Magidson;
William H. Medhurst; Ralph C.
Parent Case Text
BACKGROUND OF THE INVENTION
This application is a continuation-in-part of U.S. Ser. No.
829,794, filed Feb. 14, 1986, now abandoned.
Claims
We claim:
1. A treating process for the removal of a mold-release agent from
the surface of a molded composition, comprising the steps of:
(a) introducing the molded composition having its surface coated
with a thin film of the mold-release agent, into a treating zone,
having a controlled atmosphere;
(b) irradiating the coated surface of the molded composition with
ultraviolet radiation, said radiation having sufficient intensity
to decompose said mold-release agent to yield diverse decomposition
fragments within said treating zone;
(c) removing the diverse fragments of said mold-release agent from
said treating zone; and
(d) recovering from said treating zone a molded composition having
a clean surface, substantially free of mold-release agent.
2. The process of claim 1 wherein the mold-release agent is
paraffin wax or zinc stearate.
3. The process of claim 1 wherein the thin film of the mold-release
agent coats the surface of the molded composition to a depth within
the range from about 50 to about 500 nanometers.
4. The process of claim 1 wherein the irradiation is effected with
a laser beam.
5. The process of claim 4 wherein the laser beam is a pulsed laser
beam.
6. The process of claim 1 wherein the irradiation is effected with
a pulsed excimer laser beam.
7. The process of claim 6 wherein the pulsed excimer laser beam is
afforded by a member of the class consisting of argon fluoride,
krypton fluoride, xenon chloride, and xenon flouride lasers.
8. The process of claim 7 wherein the laser beam is afforded by a
krypton fluoride laser, having a wavelength of 248 nanometers.
9. The process of claim 7 wherein the laser beam is afforded by a
xenon chloride laser, having a wavelength of 308 nanometers.
10. The process of claim 5 wherein the laser beam is afforded by
the fourth harmonic of a neodymium-YAG laser, exhibiting a
wavelength of 266 nanometers.
11. The process of claim 6 wherein the pulsed excimer laser beam
has a pulse length within the range from about 15 to about 55
nanoseconds.
12. The process of claim 11 wherein the pulse length of the excimer
laser beam is about 45 nanoseconds.
13. The process of claim 6 wherein the pulsed excimer laser beam
has a fluence within the range from about 0.01 to about 1.0 joule
per square centimeter.
14. The process of claim 13 wherein the fluence of the excimer
laser beam is within the range from about 0.04 to about 0.70 joule
per square centimeter.
15. The process of claim 1 wherein the molded composition comprises
a plastic material.
16. The process of claim 15 wherein the plastic material is a
polyurethane or a polyester.
17. The process of claim 15 wherein the molded composition
comprises a polyester reinforced with glass fibers.
18. The process of claim 1 wherein the controlled treating zone
atmosphere is maintained under a reduced pressure.
19. The process of claim 1 wherein the controlled treating zone
atmosphere includes oxygen gas.
20. A process for preparing the surface of a molded
plastic-containing article for subsequent bonding, painting, or
other surface-modifying operation, comprising the steps of:
(a) introducing the molded article, having its surface coated with
a thin layer of mold-release agent, into preparation zone
maintained under controlled conditions of temperature and
pressure;
(b) scanning the surface of the molded article within the
preparation zone with focused pulsed ultraviolet radiation having
sufficient fluence and pulse length to effect fragmentation of the
mold-release agent;
(c) removing the fragments of the mold-release agent from the
preparaton zone; and
(d) passing the thus-prepared molded plastic-containing article,
now substantially free of mold-release agent, directly to a
selected surface-modifying treatment zone.
21. The process of claim 20 wherein the preparation zone is
maintained at substantially ambient temperature and at a reduced
pressure.
22. The process of claim 20 wherein the pulse length of the
ultraviolet radiation is within the range from about 15 to about 55
nanoseconds.
23. The process of claim 20 wherein the fluence of the ultraviolet
radiation is at least about 0.01 joule per square centimeter.
24. The process of claim 20 wherein the fluence of the ultraviolet
radiation is within the range from about 0.04 to about 0.70 joule
per square centimeter.
25. The process of claim 20 wherein the ultraviolet radiation
consists of an excimer laser beam.
26. The process of claim 25 wherein the excimer laser beam is
selected from the class consisting of argon fluoride, krypton
fluoride, xenon chloride, and xenon fluoride laser beams.
27. The process of claim 26 wherein the excimer laser beam is a
xenon chloride laser beam.
28. The process of claim 26 wherein the excimer laser beam is a
krypton fluoride beam.
29. The process of claim 20 wherein the molded article comprises
polyurethane and the mold-release agent comprises zinc stearate or
paraffin wax.
30. The process of claim 20 wherein the molded article comprises a
polyester-glass fiber composite and the mold-release agent
comprises zinc stearate.
31. The process of claim 30 wherein the polyester-glass fiber
composite additionally comprises calcium carbonate as a filler
material.
32. The process of claim 20 wherein the thin layer of the
mold-release agent coats the surface of the molded article to a
thickness within the range from about 50 to about 500
nanometers.
33. A process for providing improved bonding properties in the
surface layer of a molded composite article, comprising at least an
organic polymer component and a fiber component, comprising the
steps of:
(a) introducing the molded composite article, having its surface
layer coated with a thin film of a mold-release agent, into a
treating zone;
(b) generating ultraviolet radiation, comprising an excimer laser
beam, said laser beam having a fluence of at least about 0.15 joule
per square centimeter;
(c) directing the ultraviolet radiation upon the entire surface
layer of the molded composite article in the treating zone in a
scanning pattern, to effect substantially complete decomposition of
the mold-release agent coating thereon together with decomposition
of at least a portion of the polymer component contained in said
surface layer;
(d) removing the decomposition products from the treating zone;
and
(e) recovering the molded composite article from the
treating zone, for selected bonding processing.
34. The process of claim 33 wherein the mold-release agent is zinc
stearate.
35. The process of claim 33 wherein the organic polymer component
is a polyester material.
36. The process of claim 33 wherein the fiber component comprises
glass or graphite.
37. The process of claim 33 wherein the organic polymer component
is polyethylene terephthalate and the fiber component is glass
fiber.
38. The process of claim 33 wherein the excimer laser is a krypton
fluoride laser having a wavelength of 248 nanometers.
39. The process of claim 33 wherein the excimer laser is a xenon
chloride laser having a wave length of 308 nanometers.
40. The process of claim 33 wherein the excimer laser is a xenon
fluoride laser having a wavelength of 351 nanometers.
41. The process of claim 33 wherein the excimer laser beam has a
pulse length within the range from about 15 to about 55
nanoseconds.
42. A process for modifying the surface properties of a molded
article, having a surface layer, comprising the steps of:
(a) introducing the molded article into a treating zone;
(b) generating ultraviolet radiation, comprising a laser beam;
(c) directing the ultraviolet radiation upon the surface layer of
the molded article in the treating zone in a scanning pattern, to
effect decomposition of at least a portion of the surface
layer;
(d) removing decomposition products from the treating zone; and
(e) recovering the molded article from the treating zone.
43. The process of claim 42 wherein the laser beam is an excimer
laser beam.
44. The process of claim 42 wherein the laser beam has a fluence of
at least about 0.04 joule per square centimeter.
45. The process of claim 42 wherein the molded article comprises a
polyurethane or a polyester material.
46. The process of claim 43 wherein the excimer laser is a krypton
fluoride laser having a wavelength of 248 nanometers.
47. The process of claim 43 wherein the excimer laser is a xenon
chloride laser having a wavelength of 308 nanometers.
48. The process of claim 43 wherein the excimer laser beam has a
pulse length within the range from about 15 to about 55
nanoseconds.
Description
FIELD OF THE INVENTION
This invention relates to surface preparation, especially cleaning
or priming, by ultraviolet laser treatment of molded articles or
components, whose surfaces are to be subjected to further
processing, such as bonding or painting.
DESCRIPTION OF THE ART
Molded compositions, including plastic compositions, generally
require the use of a mold-release agent in processing to facilitate
removal from the mold. It is usually necessary to bond or paint
such molded compositions during subsequent processing steps.
Bonding to the plastic surface is poor unless the mold-release
agent is first removed.
The use of "internal" mold-release agents involves intimate mixing
of the selected agent with the substrate to be molded as
finely-divided solids, especially as powders. During subsequent
heating under pressure in the mold, much of the release agent
migrates to the surface of the mold so that upon removal from the
mold the molded object tends to be coated with a thin layer, or
film, of the selected mold-release agent. In contrast, "external"
mold-release agents are initially coated on the surface of the mold
so that, upon removal from the mold, the molded object has most of
the selected mold-release agent on its surface.
In conventional processing, mold-release agents typically include
materials such as a paraffin wax, compounds such as zinc stearate,
or polymeric substances such as silicones. Such typical
mold-release agents are customarily removed in a "wet" chemical
wash process. Such processing is energy- and labor-intensive and
employs chemicals which may constitute an environmental problem. In
the subsequent drying step, migration of remaining mold-release
agent to the surface is often experienced. This, in turn, leads to
high rejection rates in later painting or bonding operations due to
poor or inconsistent surface quality in the molded product after
washing.
Plastic materials have been replacing metals in many consumer
products at an increasing rate. Prominent usages are in
automobiles, electronic equipment, furniture, and the like. Raw
material cost as well as processing cost has been decreased. Newer
automobile models employ increasing amounts of plastic parts,
including door panels, fenders, hood and trunk sections.
Consequently, a major need exists for an improved and more economic
means for surface preparation.
While this invention relates to a novel process for surface
cleaning or priming of molded compositions in a nonchemical,
non-contact manner with little or no thermal effect, lasers have
been employed in specific surface treatments in the etching of
photoresists and related semiconductor production. Such treatments
employ high-energy excimer lasers in the far ultraviolet spectral
region.
The use of excimer lasers for photoetching of
polymethylmethacrylate films was reported by Y. Kawamura, et al.,
in Appl. Phys. Lett., vol. 40, pp. 374-5 (1982) and by R.
Srinivasan in J. Vac. Sci. Technol. B, vol. 1, pp. 923-6 (1983).
Employing the ArF laser, emitting 193 nanometer (nm) radiation,
ablated material was ejected in large fragments at low fluences
while at higher energy levels etching to a depth of about 300 nm
could be consistently realized. Similarly, Srinivasan, et al.,
reported studies on polyethylene terephthalate films in J. Am.
Chem. Soc., vol. 104, pp. 6784-5 (1982). Two related U.S. patents
have been granted in consequence of these studies; namely, S.E.
Blum, et al., U.S. Pat. no. 4,414,059, and V. I. Mayne-Banton, et
al., U.S. Pat. no. 4,417,948.
The -059 patent relates to patterning of organic resist materials
by selective ablative photodecomposition employing radiation of
wavelengths less than 220 nm. The selected power densities caused
fragmentation of resist polymer chains with immediate escape of the
volatile fragments from the resist layer. The process is stated to
be critically dependent upon the wavelength of the applied
radiation which should be less than 220 nm. The process is employed
in lithography for circuit fabrication.
The -948 patent relates to photoetching of polyesters without
causing heating or degradation of the bulk of the polymeric
material. Holes about 100 nm deep are created in the polymer film
by employing ultraviolet radiation having a wavelength less than
220 nm, the only critical parameter in this process. The process is
effective because of the ester linkage in the polyester and hence
is limited to operation on such polymers incorporating ester
groupings.
SUMMARY OF THE INVENTION
The novel process of this invention relates to the preparation of
surfaces of molded products for improved bonding and painting
performance. This invention particularly relates to the cleaning or
priming of molded plastic surfaces in a one-step, "dry" process
employing treatment with ultraviolet laser radiation.
This novel process is particularly effective in the removal of
residual films, coatings or surface layers of moldrelease agents,
such as a paraffin, a silicone, or zinc stearate, from molded
surfaces comprising plastic materials or such materials reinforced
with fibers, especially fibers of glass or graphite.
It is an object of this invention to remove both the mold-release
agent and a minor portion of the base polymeric material to provide
a clean surface for improved adhesion of paint or bonding of other
surfaces to said surface within a time period such that no
mold-release agent can migrate to the surface prior to said
improved bonding operation.
It is another object of this invention to remove both the layer of
mold-release agent and a larger portion of the base polymeric
material to expose some glass fiber surfaces for especially
improved bonding to the surface.
A preferred embodiment of this invention includes the steps of:
(a) introducing the molded composition having its surface coated
with a thin film of the mold-release agent, into a treating zone,
having a controlled atmosphere;
(b) irradiating the coated surface of the molded composition with
ultraviolet radiation, said radiation having sufficient intensity
to decompose said mold-release agent to yield diverse decomposition
fragments within said treating zone;
(c) removing the diverse fragments of said mold-release agent from
said treating zone; and
(d) recovering from said treating zone a molded composition having
a clean surface, substantially free of mold-release agent.
The ultraviolet radiation source is preferably a pulsed excimer
laser, such as a krypton fluoride (248 nm) or xenon chloride (308
nm) laser, scanning the surface of the molded composition at a
fluence and pulse length (one or more pulses) to provide sufficient
energy density to fragment the mold-release agent coated thereon.
In general, the fluence will range between about 0.01 and about 1.0
joule per square centimeter to remove a film layer, including
mold-release agent, up to about 500 nm thick. Greater fluences may
be employed where necessary.
DESCRIPTION OF THE INVENTION
This invention relates to a novel process for ultraviolet laser
treatment of molded surfaces to improve significantly the bonding
strength and adherence of coatings thereto.
One embodiment of the process provides a one-step "dry" process for
substantially complete removal of mold-release agent from the
subject molded surface when sufficient fluence and time are
employed in the ultraviolet irradiation to decompose the
mold-release agent and eject the evaporated decomposition
fragments. The term "mold-release agent" as used herein is
understood to include contaminants and other adventitious
substances which interfere with bonding to the polymer, or plastic,
surface. As used herein, use of the term "ultraviolet radiation"
conforms to the definition found in H. Bennett, Concise Chemical
and Technical Dictionary, (Chemical Publishing Co., New York, 1974)
as "light waves shorter than the visible blue-violet waves of the
spectrum, having wave lengths of 136-400 .ANG.", or 13.6-400
nm.
In the process of this invention the shape of the molded object
remains unchanged. Problems related to solvent removal of
mold-release agents are avoided. These include mold-release agent
surface migration during drying; exposure to harmful chemicals;
trapped solvent, chemicals or water when molded parts are complex
in shape.
The process operates by evaporation or decomposition of
mold-release film from the molded object surface in a very short
time. Associated plastic material in the surface film, or layer, is
also removed. The result is greatly improved bonding to the clean
molded surface, even though additional mold-release agent may be
present internally with the potential for eventual migration to the
surface.
In one particular embodiment of this invention the molded
composition, having its surface coated with a thin film of
mold-release agent, is placed in a controlled treating zone and
irradiated wtih ultraviolet radiation having sufficient intensity
(joules/cm.sup. 2/sec) to remove or decompose the moldrelease
agent. Decomposition fragments are removed from the controlled zone
and the molded composition is thereafter recovered. Its surface is
now clean, smoother, and substantially free of mold-release
agent.
When removed from the mold, the workpiece is generally coated with
a film of mold-release agent, such as a paraffin oil or wax,
silicone, or zinc stearate, having a depth, or thickness, within
the range from about 50 to about 500 nm. Most often the film
thickness ranges from about 100 to about 300 nm.
Irradiation is usually effected with an ultraviolet laser beam,
preferably with a pulsed laser beam of the excimer type. Among
suitable excimer lasers are argon fluoride (193 nm), krypton
fluoride (248 nm), xenon chloride (308 nm), an xenon fluoride (351
nm). Even shorter wavelength lasers should be very effective, but,
because of the expensive optics involved with the low wavelength
argon fluoride laser, preferred excimer lasers are the krypton
fluoride and xenon chloride lasers. Among other lasers radiating in
the ultraviolet region, the fourth harmonic of the neodymium-YAG
(266 nm) laser is effective and therefore is a preferred laser.
In gneral, pulse length should be less than 1.0 microsecond and
preferably may vary from about 15 to about 55 nanoseconds (ns). In
one embodiment a pulse length of about 45 ns is preferred. Fluence
may vary from about 0.01 to about 1.0 joule/cm.sup.2 or even
greater, preferably from about 0.04 to about 0.70 J/cm.sup.2.
Although pulse lengths in the range from about 15 to about 55 ns
have great utility in this process, even shorter pulse lengths can
be very effective. With such shorter pulses, the pumping rate
exceeds the energy loss rate (by thermal diffusion or relaxation of
electronically excited states) so that energy is more efficiently
accumulated in the irradiated zone.
Because of their growing use in many types of consumer products,
the typical molded composition will comprise a plastic material.
Typical plastics include polyurethanes, which may contain glass
particles, and polyesters, the latter often being reinforced with a
fibrous component such as fibers of glass or graphite. A preferred
molded composition comprises a polyester, polyethylene
terephthalate (PET), reinforced with glass fibers. This latter
composition may also contain an inorganic filler material, such as
calcium carbonate.
Generally, the controlled treating zone may be either an open area
or an enclosed volume maintained under a partial vacuum or under a
gas mixture which may include a partial pressure of oxygen gas. The
vacuum assists in rapid removal of decomposition fragments (as
vapor) of the mold-release agent and associated polymer material.
It is believed that the presence of oxygen may serve to assist in
rapid fragmentation by avoiding recombination of radicals whose
formation has been induced by the ultraviolet radiation.
The molded articles with which this invention is concerned may
exhibit a surface roughness amounting to some 2000 nm. when
measured as the extremes of peak to valley. While this degree of
roughness is not great for the purposes of this invention, it is
far outside the permissible range for the utility disclosed in the
prior art. For semiconductors or other circuit fabrication,
surfaces must be extremely flat and smooth and etched holes must be
defined by sharp boundaries, and steep, perpendicular walls. Such
surfaces are not pertinent to the preferred operations of this
invention. The process of this invention is concerned with
selective removal of surface layers over a wide area, modifying the
surface chemistry, rather than precise machining of micro areas to
produce highly defined micro features.
Because the process of this invention does not require the extreme
surface smoothness of the prior art, less energetic lasers than the
ArF excimer laser are preferred because of their lower expense
coupled with highly effective characteristics. In exposing the
entire surface of the molded object to ultraviolet radiation, a
scanning action must be employed. Depending upon the available
apparatus configurations, the object surface may be scanned by a
programmed movement of either the workpiece or the laser beam.
Although a minor portion of the radiation energy will typically be
absorbed by the molded article, very little effect on bulk
temperature has been observed so that the process of this invention
is normally conducted at substantially ambient temperature.
However, temperatures at the point of incidence may be momentarily
higher to effect decomposition or evaporation of the surface film
of mold-release agent or polymer material.
By exposing the molded surface to an excess of ultraviolet
radiation energy over that required for decomposition or
evaporation of the mold-release agent, some selective removal of
the plastic substrate can occur. When employing a composite of
fibers, such as glass fibers, and an organic polymer, such as PET,
in the molded article, the process of this invention can
selectively strip away portions of the organic polymer to expose
clean glass surfaces. Such surfaces exhibit excellent bonding
strength, as, for example, for various paints, including acrylic
lacquers, and bonding agents.
Surface cleaning, for the purpose of effecting improved bonding of
plastic substrates, need not expose glass fibers by etching so
deeply as to remove all of the plastic material. However, it does
appear necessary to etch deeply enough to have removed
substantially all of the mold-release agent. Shear tests run after
permitting sufficient time for migration to the surface of internal
concentrations of mold-release agent have given poor results which
may suggest that bonding operations should occur as soon after
laser cleaning of the surfaces as is practicable. It is thus
preferred that the treatments be conducted in a substantially
concerted manner.
The following examples are illustrative, without limitation, of the
novel process of this invention.
EXAMPLE I
An excimer laser beam was generated with a LambdaPhysik laser unit
(Models 102 and 201E). The laser pulse width was 15-25 ns. The
beam, having a rectangular cross-section of 1 cm.times.3 cm was
first passed through a copper mask, having a 1 cm.times.1 cm square
hole. The beam then passed through an attenuat and a Suprasil beam
splitter. The reflected beam as directed onto a Joule meter which
was connected to a microcomputer which measured the laser pulse
energy and controlled the firing of the laser. The transmitted beam
was directed onto the workpiece. The Suprasil beam splitter
transmittance spectrum was recorded with a UV-Visible spectrometer
in the 180-400 nm spectral range.
Plastic sample pieces were prepared from the surface of a Pontiac
Fiero door panel (polyurethane with glass platelet reinforcement).
Peak to peak roughness was greater than 2000 nm. Each sample piece
(2.5 cm diameter) was placed in the beam path and irradiated with
1-100 pulses from the selected excimer laser. In most cases only
one pulse was necessary for surface cleaning. Selected lasers
operated at wavelengths of 193 nm (ArF), 248 nm (KrF), 308 nm
(XeCl), and 351 nm (XeF). Thereafter, the entire sample surface was
painted with red acrylic lacquer (Dupont Lucite) mixed with a paint
thinner.
The paint first stuck to both the exposed (primed) and unexposed
areas. After drying for 24 hours, the paint in the unexposed areas
lifted off with masking tape. The paint in the exposed areas could
not be removed with masking tape.
The threshold laser fluence, above which paint adhered to the
plastic surface was determined for each laser system. Results are
presented in Table I.
EXAMPLE II
The laser priming procedure of Example 1 was employed with ArF (193
nm) and KrF (248 nm) lasers. Their fluences were much higher than
the threshold fluence for cleaning the plastic. No lens was
required to increase the fluence at the sample surface. Energy in
the beam was attenuated with an appropriate combination of quartz
plates and Schott cutoff filter WG-230. The samples were placed in
the reflected beam when low laser fluence was required at the
sample surface. Similarly, the sample was placed in the transmitted
beam when a higher fluence was required. After radiation, the
sample surfaces were painted and treated as in Example I. Effects
are presented in Table I.
EXAMPLE III
The laser priming procedure of Example I was employed with XeCl
(308 nm) and XeF (351 nm) lasers. Because the laser beam output was
lower than the threshold fluence, the attenuator was replaced with
a lens. The fluence at the sample surface was varied by changing
the distance between sample and lens (moving the sample). The
sample surfaces were painted and treated as in Example I.
Paint adhered to the exposed areas at high fluence levels but there
was some evidence of a heat affected zone on the sample surface.
Effects are presented in Table I.
EXAMPLE IV
The output frequency of the Nd-YAG laser (Quanta Ray Model DCR-30A)
was quadrupled using KDP nonlinear crystals to generate laser
radiation at 266 nm. Following radiation of the sample, the
procedure of Example I was followed.
Paint adhered to the irradiated areas at all fluences higher than
the threshold value, included in Table I.
EXAMPLE V
The laser priming procedure of Example I was employed with a XeCl
(308 nm) laser in irradiating a glass-filled polyurethane coated
with either paraffin wax or zinc stearate as mold-release agent.
Release agent removal was essentially complete when either sample
was exposed at a fluence of 0.47 J/cm.sup.2 and a pulse length of
45 ns.
EXAMPLE VI
The procedure of Example I was followed with a XeCl (308 nm) laser
in irradiating a polyester-glass composite coated with zinc
stearate. Release agent removal was essentially complete when the
sample was exposed at a fluence of 0.64 J/cm.sup.2 and a pulse
length of 45 ns.
At a fluence of 0.8 J/cm.sup.2 selective removal of polyester from
the composite was observed, leaving glass fiber surfaces unaffected
and exposed. Greater exposure was observed at still higher
fluences.
EXAMPLE VII
The procedure of Example I was followed employing sample pieces of
sheet molding compound (SMC) taken from a Pontiac Fiero hood panel
(roughly equal weight parts of polyester-glass fiber-calcium
carbonate). Etch rate per pulse were measured as selected fluences
with the observed results as presented in Table II.
EXAMPLE VIII
Hood panel samples, as in Example VII, were treated with laser
radiation at selected fluences and thereafter subjected to a
bonding test to determine the effectiveness of surface
cleaning.
Laer-cleaned sample pieces were bonded together employing a
diol-diisocyanate bonding adhesive (Pliogrip-Ashland Oil Company)
at ambient temperature to acheive a polyurethane bond. The mixed
adhesive was spread over a 1".times.1" surface area of
laser-treated samples. Glass beads, having 0.02" diameter, were
placed in the adhesive to control bond thickness. Two similarly
treated samples were then pressed together uniformly and allowed to
stand for about 15 minutes. Thereafter, excess adhesive was removed
an the samples were baked in an oven at 250.degree. F. for 1
hour.
Shear strength was determined at room temperature and at
180.degree. F. employing an Instron tester, pulling at a rate of
0.5 inch/minute.
Test results at 180.degree. F. are presented in Table III. At room
temperature all samples failed at high pressures by a cohesive
plastic break in the bulk of the sample piece. At 180.degree. F.,
three types of breakage were observed, including an interface break
between sample piece and adhesive, and a cohesive glue break within
the adhesive material, leaving the bond between sample piece and
adhesive intact. Tests with the ArF laser indicated failure at the
interface between sample and adhesive. Similar results were
obtained when bonding was delayed for 1 week after laser
treatment.
Molded plastic surfaces for use in this invention are typically
exemplified by (1) molded polyurethane articles, generally
including glass platelets as filler material, or (2) molded
polyester articles, generally including glass fibers and an
inorganic filler such as calcium carbonate.
In a preferred process mode for preparing a polyurethane plastic
surface for subsequent painting, an excimer laser beam at 248 nm is
generated employing krypton fluoride. The resulting laser beam is
then passed through attenuators and is split into two beams using a
Sprasil beam splitter. A first beam is directed onto a Joule meter.
A second beam is directed onto the molded plastic surface in a
scanning manner. Firing of the laser and measurement of the pulse
energy is effected with a microcomputer. The molded plastic surface
is exposed to 5 laser pulses having a pulse width of about 20 ns
and a fluence of about 0.045 J/cm.sup.2. The treated surface is
painted within about 3 hours after the laser beam priming
operation.
More severe surface treatment is required in preparing a polyester
plastic surface for subsequent bonding, employing a KrF excimer
laser beam at 248 nm. The surface is exposed to 2 laser pulses
having a pulse width of about 20 ns and a fluence of about 0.18
J/cm.sup.2. This treatment is sufficient to remove the plastic
layer that is especially contaminated with mold-release agent but
does not remove all of the polyester component to expose the glass
fibers. Bonding of treated surfaces is effected promptly, employing
a polyurethane adhesive.
Greater laser wavelengths are preferred when the exposure of glass
fibers is desired. For example, sheet molding compound is prepared
for subsequent bonding employing a XeCl excimer laser (308 nm) at a
fluence of about 1.0 J/cm.sup.2 and a pulse width of about 20 ns.
Only 1-2 pulses are required to expose glass fibers, having clearly
removed both polyester and calcium carbonate filler. At this
wavelength, the optical absorption coefficient is in the range from
about 10.sup.2 -10.sup.3 /cm so that each laser pulse removes 1-3
microns of material.
TABLE I ______________________________________ Threshold Fluence
Values.sup.a vs. Laser Wavelength Laser Wavelength Threshold
Fluence Paint Type (nm) (J/cm2) Adherence
______________________________________ ArF 193 0.013 Excellent KrF
248 0.045 Very good Nd--YAG .sup. 266.sup.b 0.05-0.20 Good XeCl 308
>0.5 Good, but some evidence of heat effect. XeF 351 >1.0
Good, but some evidence of heat effect.
______________________________________ .sup.a Polyurethane plastic
panel, containing glass platelets. .sup.b Fourth harmonic.
TABLE II ______________________________________ Etch Rate vs. Laser
Wavelength Laser Wavelength Fluence Pulses Etch Rate Type (nm)
(J/cm.sup.2) (no.) (microns/pulse)
______________________________________ ArF 193 0.44 10 0.08 100
0.08 KrF 248 0.40 10 0.14 100 0.13 XeCl 308 0.44 10 0.65 100 0.17
0.80 10 2.40 100 0.14 1.1 10 1.10 XeF 351 0.77 10 0.65 100 0.08
2.04 1 7.50 5 7.60 ______________________________________
TABLE III ______________________________________ Shear Strength
Tests Laser Wavelength Fluence Pulses Failure.sup.a Failure Type
(nm) (J/cm.sup.2) (no.) (psi) Type
______________________________________ ArF 193 0.15 1 338 .sup.b
KrF 248 0.18 2 347 .sup.c 6 390 .sup.d 0.41 1 432 .sup.d 0.43 5 455
.sup.c KrF.sup.e 248 0.43 5 473 .sup.b XeCl 308 1.50 1 473 .sup.c 5
510 .sup.b,c ______________________________________ .sup.a Shear
test run at 180.degree. F. .sup.b Interface break between sample
piece and adhesive. .sup.c Cohesive plastic break in the bulk of
the sample piece. .sup.d Cohesive glue break within the adhesive
material. .sup.e Sample pieces bonded 7 days after laser
cleaning.
* * * * *